Conventionally, a silencer for suppressing exhaust noise is known in an exhaust system of an internal combustion engine. The silencer, for example, comprises a casing, an inlet pipe for introducing an exhaust into the casing, and an outlet pipe for discharging the exhaust out of the casing. A valve body is provided at an opening on an exhaust inflow side of the outlet pipe, so as to open and close the opening.
Patent Document 1 discloses a silencer comprising a first exhaust outlet pipe (outlet pipe) and a second exhaust outlet pipe having a smaller diameter than the first exhaust outlet pipe, and the second exhaust outlet pipe is joined in the middle of the first exhaust outlet pipe. The silencer, when a rotation speed of an internal combustion engine is low and emissions are low, introduces the exhaust into the second exhaust outlet pipe having a smaller diameter, and discharges the exhaust from the second exhaust outlet pipe to an outside of the silencer through the first exhaust outlet pipe, thereby to improve silencing performance.